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A ChatGPT clone, in 3000 bytes of C, backed by GPT-2

Dec 12, 2024 - nicholas.carlini.com
The article describes a compact, dependency-free implementation of the GPT-2 model in C, which is approximately 3000 bytes in size. This implementation includes loading the weight matrix and BPE file from TensorFlow files, tokenizing input using a byte-pair encoder, performing matrix math operations, defining the transformer architecture, and un-tokenizing output with a BPE decoder. The program is optimized to run GPT-2 Small efficiently on modern machines, though it produces low-quality output. The implementation includes features like KV caching and an efficient matrix multiplication algorithm, with optional OpenMP parallelism, and is capable of running on systems with limited resources, provided ASCII input is used.

The article provides a walkthrough of the C code, detailing the construction of a minimal matrix math library, fast matrix multiplication, neural network layers, and the transformer architecture. It also explains the byte pair encoding process and the loading of model weights and BPE vocabulary. The author highlights the simplicity of neural networks and how the implementation captures the essence of state-of-the-art models in a compact form, despite the challenges posed by the original file formats and encoding schemes.

Key takeaways:

  • This C program is a compact, dependency-free implementation of the GPT-2 model, optimized to run efficiently on modern machines.
  • The program includes a basic matrix math library, fast matrix multiplication, neural network layers, and a transformer architecture, all implemented in a minimal amount of code.
  • Byte pair encoding is used to tokenize input text, allowing the model to handle a wide range of words and characters efficiently.
  • The program demonstrates the simplicity of neural networks by distilling complex machine learning concepts into a few thousand bytes of C code.
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